I use the little Bontrager ION and FLARE RT series (like the cubes, but a little newer). USB re-chargeable, and irregular flashing patterns to supposedly grab attention better. Kind of pricey.
If I'm actually riding in darker conditions, I use a NiteRider Lumina 1200 boost. That thing is basically good enough to do off road riding in the dark at its highest power setting, but it also has a nice flashing setting at lower power.